Since 1980, maize and wheat crops in many places have been increasingly exposed to extreme heat during sensitive flowering phases that can damage them and cause harvests to fail. That’s according to scientists at Stanford University, California, who predict that this problem will increase for these crops, and also hit rice. In fact, the area of maize and rice hit by such deadly heat is set to expand more quickly through to the 2050s. “Crop breeders need to think carefully about how to incorporate heat tolerance, particularly during the flowering period, into wheat, maize and rice,” Stanford’s Sharon Gourdji told me.

“The net impact of all these factors is the golden question, but notoriously difficult to model,” Sharon explained. “Also, the most relevant of these factors, and the associated adaptation measures, differ by location and crop. Therefore most modelling studies to date look at net impacts on just a given region, or type of cropping system.” To make worldwide predictions that could help secure our future food supply, Sharon’s team had to concentrate on a smaller, simpler issue. “We focused on extreme heat during flowering,” Sharon said. “This is one aspect of global environmental change that could be particularly risky for crops regardless of other more gradual changes that are taking place simultaneously.”

Crossing the line

The percentage of global harvested area with at least five reproductive days over the critical temperature threshold for maize, wheat, rice and soybeans. Values are shown historically by year from 1980 to 2011, with associated trend lines. Statistically significant past trends are thicker. Future predicted values by decade until the 2050s represent the range of projections from the 37 CMIP5 models. Red points use the main critical temperatures, while orange points show what happens if crops have 1°C higher critical temperatures. Growth in this proportion accelerates in the future for rice and maize. Dotted lines show the range of results from individual models. Credit: IOP Publishing, via Creative Commons license, see citation below.

The researchers looked at past trends and future projections for maize (also called corn), rice, soybeans and wheat, the world’s four most common crops. First they found the world’s major production areas using maps of the harvested areas for each. They also collected figures published by other scientists for the critical high temperatures beyond which each crop is damaged during the flowering stage. For wheat the threshold is 34°C, maize is 35°C, rice 36°C and soybean 39°C.

But they couldn’t find good estimates on when each crop flowered. So instead they estimated timings on a simple proportion of the season between sowing and harvest dates that were available for each growing location and crop. “We identified a 30-day window between sowing and harvest that should include flowering,” Sharon said. “Then we simply counted the number of days over the critical high temperature threshold during the 30-day window for each crop, location and year in the study.”

In a paper published last week in Environmental Research Letters, they show that from 1980-2011 maize has increasingly crossed its threshold in many places across the globe. Maize had the highest exposure in the 2000s, with 15% of global harvested area, mainly in or near tropical regions, exposed to at least five days over the critical temperature during the estimated flowering period. Wheat has mainly crossed the threshold more in Central and South Asia and South America, while soybeans and rice saw little sustained exposure.

In projections from climate models all four crops will pass the threshold more widely in any given year. Sharon’s team estimates that by the 2030s, 31%, 16%, and 11% respectively of the global harvested area of maize, rice, and wheat will be exposed to at least five flowering days over the threshold. The growth in the total area of maize and rice exposed accelerates, reaching 44% and 27% respectively by the 2050s.

While this study doesn’t make any links to the amount of each crop finally harvested, its findings underline the need for farmers to prepare to adapt. “National-level governments, particularly in tropical countries that grow a lot of wheat, maize and rice, need to think proactively about how to help farmers adapt to expected climate conditions in the next few decades,” Sharon said.

The average change across the climate models Sharon’s team uses from the 2000s to the 2030s (left column) and 2050s (right column) in flowering days over the critical temperature threshold for maize, wheat, rice and soybean crops. Credit: IOP Publishing, via Creative Commons license, see citation below.

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